The present invention relates generally to the transmission and reception of a telephone signal between a remote battery powered cordless handset unit and fixed hard wired base unit, and in particular, to a microprocessor based digital cordless telephone apparatus capable of transmitting both digitized voice data and digitized command data between a hand unit and a base unit. In the present cordless digital telephone apparatus the handset and base unit communicate with one another using FSK modulated digital signals transmitted on an RF carrier in the 902-928 MHz band.
The function of the conventional prior art cordless telephone is to provide the user with the ability to freely move about while speaking on the telephone without the hindrance of being "tied" down by the coiled cord connecting the handset to the conventional telephone set. The typical prior art cordless telephone comprises a base unit which is physically connected to the user's telephone company lines and a hand-held handset unit. The physical hard wire connection between the conventional handset and telephone set is replaced by an radio frequency (RF) link, usually in the 46 and 49 MHz bands. The spoken voice is usually communicated between the base and handset by first converting the user's voice into an analog electrical signal and modulating the signal using an RF carrier for radio transmission to the receiver, typically through the use of a Narrow-Band Frequency Modulation (NBFM) technique. At the receiver the modulated analog voice signal is demodulated and directed to a speaker through which the voice is heard. The various command functions which must be communicated between the handset and the base are instead communicated in the digital format. To accommodate both formats of data the digital command signal is usually modulated as either a 600 Hz or 1 kHz square wave and transmitted as an "in-band" signal on top of the analog voice signal.
One shortcoming of the in-band transmission of digital command data is that the command data is part of the voice data signal and thus is inherently audible to the user when the analog voice signal is demodulated and listened to. Moreover, the command data in addition to being audible to the user is also transmitted very slowly precluding implementation of channel monitoring. Another limitation inherent in transmitting an analog voice signal using the NBFM technique is the often occurring static, interference and otherwise poor reception which accompanies the transmission and reception of the analog voice signal. While some recent cordless telephone designs have sought to include "enhanced" circuitry to improve the transmission and reception of the analog voice, hoping to obtain a "corded" sound quality, many are still subject to troublesome static and interference.
One way in which the prior art has attempted to overcome static and interference has been by providing the user with the ability to select among several different RF channel frequencies in hopes of finding a "clearer" RF link. While the ability to change channels is useful, prior art devices require that channel changing be done manually by the user who must elect to change channel based upon how much interference he or she perceives. Should the interference be in the transmission from the handset only, it will usually not be heard by the handset user, but may be very annoying to the party at the other end of the line.
When a handset is transmitting command data to a base, such as when the user accesses an outgoing line and dials a telephone number, it is essential that the base receive the complete command if the cordless telephone is to operate as intended. Static and interference in the RF link may obscure the flow of command data and cause the complete command data not to be received. The user will learn of the missing or lost command data only as a result of the device's failure to respond to the user's command request or the execution of an unintended command.
Moreover, while virtually every RF communication device has a physical range limitation beyond which a transmitter and receiver cannot communicate with one another, the implication of this limitation represents a significant shortcoming in the operation of a cordless telephone. For example, when the handset is physically located at the outer edge of the communication range the received signal is often weak and interference from other signals may result in intermittent loss of the RF link when a conversation is in progress In addition, when the handset unit is in a "standby mode" awaiting receipt of an incoming telephone call and the user moves out of range from the base unit, the user is unable to receive an incoming telephone call from the base unit and more importantly is totally unaware of being out of range unless an attempt is made to use the handset at which time the out of range condition would be discovered due to the inability to establish communication with the base unit
When the prior art handset unit is in a standby mode it must nevertheless be able to receive an incoming call from the base unit and accordingly at least the receiver circuits of the handset must remain energized. The need to maintain power to the receiver as well as other portions of the handset, even when in a standby mode, places a continuing power drain on the handset batteries and will ultimately serve to deplete the handset battery charge necessitating that the handset be returned to the base unit for recharging even if no telephone conversations have been made or received.
Accordingly, the present invention seeks to address the foregoing limitations of the prior art cordless telephone by providing a cordless telephone apparatus which comprises a base unit and handset unit which each transmit and receive digitized voice signals toward providing true noise free conversations with significantly greater immunity to static and interference, and with increased range.
Another object of the present invention is to provide for the combined communication of digitized voice signals and digital command signals toward the wireless transmission and reception of such signals on an RF band of 902-928 MHz taking advantage of revised FCC regulations which have allocated portions of this band for just such an application.
Moreover, the present invention seeks to provide for the seamless intermixing of digital command data into the stream of digital voice data flowing between the base unit and the handset.
It is an associated object of the present invention to provide a means by which transmitted digital command data is received and captured by the receiving unit from the stream of incoming digital data and replaced with a quiet sequence so as to prevent the command data from causing an otherwise undesirable audible sound to be heard by the user.
It is a still further object of the present invention to implement a command data protocol incorporating positive acknowledgment with retransmission technology in order to insure that each transmitted command is acknowledged when received and if lost or damaged is repeated until acknowledged all before further commands are transmitted.
It is yet another object of the present invention to provide a command data protocol which incorporates a randomly generated security code which prevents an unintended person from gaining access to the user's telephone line and placing unauthorized telephone calls.
It is additionally an object of the present invention to provide for the selection and changing of RF channel frequencies without intervention by the user in response to the automatic detection of interference in the RF link or a total loss of the RF link.
Another object of the present invention is to provide twenty frequency channels for operation of the handset at operating frequencies of 925.5 to 927.4 MHz and twenty paired frequency channels for operation of the base unit where such channels are paired in a manner which permits rapid channel scanning.
It is yet a further object of the present digital cordless telephone apparatus to provide for the transmission of a link check command signal by the base unit and the acknowledgment of receipt by the handset unit in order to continually monitor the viability and existence o the RF link while a call is in progress as well as to detect an out of range condition toward signaling the user at the handset of the inability to make or receive a telephone call.
Yet another object of the present invention is to provide a means by which the handset and base unit can communicate data between one another through a physical connection when the handset unit is in its storage cradle within the baseset unit, thereby precluding the need to modulate the recharge power source as a way of establishing communication between the handset and the base unit when in storage.
It is yet a further object of the present invention to provide a power saving mode wherein the handset unit automatically powers down when a call is not in progress and periodically automatically awakens to check for the presence of an incoming call, incoming link check command signals or the actuation by the user of the keypad, thereby serving to extend handset battery life.
An additional object of the present invention is to provide for the scrambling of the digital voice signal prior to transmission and for the unscrambling of the digital voice signal upon reception in order to minimize the possibility that the telephone conversation may be monitored by unintended persons listening to the RF channel frequency.
These and other objects of the invention will become apparent in light of the present specification and drawings.